Field of the Invention
This invention relates to improvements in the suspension system or a vehicle, and is specifically related to controlling the disposition of the vehicle body relative to the ground when the vehicle is subject to variations in the contour of the surface being traversed.
Description of the Background Art
In recent times, there has been a trend towards resilient sprung suspension systems incorporating variable damping and spring rates in an attempt to improve vehicle stability and reduce movement of the vehicle body relative to the surface being traversed.
A range of suspension systems known as `active` and `semi-active` suspensions for vehicles have been trialed including systems operating on the basis of compression and/or displacement of fluids, and such systems currently in use incorporate a pump, to maintain the working fluid at the required pressure and effect the high speed distribution thereof, and sophisticated control mechanisms to regulate the operation of the suspension system in accordance with sensed road and/or vehicle operating conditions. These known systems incorporating pumps and electronic control systems, which both usually operate continuously while the vehicle is in operation, are comparatively expensive to construct and maintain, and require a substantial energy input. They therefore are finding limited acceptability in the vehicle industry.
There is previously published an International Patent Application (International Publication Number WO 93/01948, International Application Number PCT/AU92/00362 and dated Feb. 4, 1993) which discloses a `passive` hydropneumatic vehicular suspension system. This disclosed passive suspension system has many of the advantages of `active` or `semi-active` suspension systems, whilst avoiding the complexity and expense of such systems, thereby making it more acceptable to the automotive industry.
In the suspension system disclosed in said patent, a front wheel ram and the diagonally opposite rear wheel ram have the upper chamber of the front ram interconnected with the lower chamber of the rear ram and the lower chamber of the front ram interconnected to the upper chamber of the rear ram. Similarly the respective chambers of the other front ram and rear ram are likewise interconnected. There is thus provided two individual fluid circuits, each comprising a front ram and a diagonally opposite rear ram. Each of the conduits interconnecting the respective upper and lower chambers normally has at least one conventional pressure accumulator in communication therewith. The two circuits are interconnected to a pressure balancing device which is arranged to maintain a substantially equal pressure in the two circuits, as is described in detail in the previously referred to International Patent Application No. WO93/01948.
This prior proposed vehicle suspension system obviates the use of ordinary springs (eg. coils, leaf, or torsion bar springs) as well as conventional telescopic dampers (commonly referred to as shock absorbers) and roll or sway stabiliser bars.
Springing or resilience is provided by way of the gas filled accumulators with damper valves located in the mouths of the accumulators. Conventional vehicles fitted with accumulator springs are known to provide good comfort levels when traversing low amplitude ground surfaces at most speeds. However, accumulators gassed to provide a soft ride also tend to induce and exaggerate unwanted roll and pitch motions when used without roll or sway stabiliser bars. Most hydropneumatically suspended vehicles are therefore normally provided with roll or sway bars made of spring steel which mechanically and transversely interconnect the two wheels of each axle thereby limiting roll but not pitch movements.
In the suspension system described above, (Patent # WO 93/01948), excessive roll movements are resisted and controlled hydropneumatically without roll stabiliser bars and the amount of roll permitted is defined by a function of the ratio of the rams' cylindrical bore diameters (of the diagonally opposite rams) to the ram rod diameters, and with regard to their stroke lengths and with regard to the amount of gas within the various accumulators of the suspension system.
It is also to be noted that the type of wheel geometry and the location and design of various components may give some components a mechanical advantage over others thereby providing for example, an appropriate but different amount of roll stiffness at the front relative to the rear of the vehicle which to an extent defines whether the vehicles under or oversteers when cornering.
In conventional vehicles, roll forces are resisted by the roll or sway bars, i.e. transverse mounted, formed spring steel bars which must be deformed in torsion for any body roll to occur. Conversely, pitch motion in the longitudinal plane is normally only partially resisted by the design of the suspension geometry with spring resonances being avoided through the appropriate selection of front and rear spring and damper rates without the need for any direct acting mechanical equivalent of the roll bar. This is because the pitching actions in the longitudinal direction are less severe than the transverse rolling actions.
It has been found that the system previously disclosed provides adequate comfort, stability and relatively consistent wheel loading irrespective of relative wheel travel positions during many manoeuvres such as axle articulations and single wheel inputs, however, the magnitude of pitch and roll control is governed by the same components and the effective linear stiffness of each wheel in relation to the vehicle body in either pitch or roll is typically the same. In long wheel based vehicles this translates to stiff pitch characteristics in relation to roll. In short wheel based vehicles, pitch and roll stiffness become closer in magnitude. As most vehicles are considerably narrower than they are long, and due to other geometric effects it has been found that roll is more difficult to control than pitch as noted above. Indeed, when the suspension system is designed to adequately contain roll movement the pitch motions may be consequently over compensated for in the system. This may be further clarified as follows:
In order to contain high roll forces resulting from a high centre of gravity with respect to the relatively closely located rams (in the transverse direction), it is necessary to supply rams with a greater difference in rod and bore diameters. This therefore may automatically generate an unnecessary amount of pitch resistance or control in the longitudinal direction and this can lead to harshness of ride quality in some conditions. In particular it has been found that while body disturbance due to axle articulation movements and single wheel inputs is minimised, road surfaces that give rise to double wheel inputs on a single axle (such as `speed humps`) or sinusoidal road profiles can upset the previously disclosed suspension system. Typically this occurs when a vehicle's wheel base length approximates to half of the spacing of the humps disposed along the road surface. In order to traverse this kind of road surface smoothly (without excessive pitch motions being induced) both axles need to become independent in their motion, however the previously disclosed interrelated hydropneumatic system interprets these motions as high speed pitch movements and therefore attempts to resist them as though they were unwanted pitch motions. This type of high speed pitch resistance and over compensation manifests itself as an inappropriate pitch harshness which can become additionally uncomfortable when the vehicle moves over repeated bumps or dips causing increasingly exaggerated and inappropriate resonant responses.